Couplers are widely used in radio frequency and microwave systems to allocate and integrate signal power and to sample and detect power in a balance device of amplifier, phase shifter, filter, etc. A typical coupler is actually a four-port network dividing an input signal in a specific frequency range into two output signals the power of which is in a specific ratio. There are numerous types of couplers with different natures including a coupling line directional coupler arranged on a Printed Circuit Board (PCB). FIG. 1 illustrates a general structure of a coupling line directional coupler in the prior art. As illustrated in FIG. 1, a primary signal line 11 is provided with two ports, a first port and a third port, and a coupled signal line 12 is also provided with two ports, a second port and a fourth port. When a signal is input from the first port to the primary signal line 11, a coupled signal may be generated on the coupled signal line 12 due to electromagnetic induction and be output from the second and fourth ports.
A weak coupler (with a coupling degree of 30 dB) in the directional coupler is typically used to detect a level of a high-power signal between a Power Amplifier (PA) and an antenna feed system. “Directional” refers to the coupled signal is stronger at the second port than at the fourth port. If the coupling degree of the weak coupler is known, then the level of the input signal at the first port may be calculated simply by detecting the power level of the output signal at the second port. The fourth port is an isolation terminal where a useless signal is output and which is grounded via a match absorption load.
A highly directional (or high-directivity) weak coupler in a wireless access system is primarily used in power detection and standing wave detection circuits of an antenna feed system. FIG. 2 illustrates a block diagram of a circuit of a high-directivity coupler used for antenna feed standing wave detection in the prior art, where resistors R1 and R2 are match resistors. Its detection principle lies in that a forward coupler 21 and a forward power detection circuit 22 detect forward power while a backward coupler 23 and a backward power detection circuit 24 detect backward power, and the difference between the forward power and the backward power is calculated as a return wave loss which is converted into a standing wave of the antenna feed system by a formula. In order to improve a precision of detecting the standing wave of the antenna feed system and hence reduce an error ratio, directivity of the directional coupler has to be improved as much as possible, typically up to 28 dB, and a theoretical derivation thereof is well known in the art and it is not detailed here.
In order to implement high directivity of the weak coupler, it is generally required that the primary signal line and the coupled signal line be arranged in the same medium with an isotropic dielectric constant and magnetic leakage ratio.
If they are not arranged in the same medium, then there may be different phase velocities of odd and even modes (that is, an inappropriate ratio between mutual capacitance and mutual inductance), and in this case, manufacturers may adopt a modified structure to make the phrase phase velocities of the odd and even modes equal, thereby improving directivity. A directional coupler in the prior art is described briefly.
A first prior art relates to a metal rod coupler with the medium of air illustrated in FIG. 3. The medium of air is a medium with a uniform electromagnetic nature, and both a primary signal line 31 and a coupled signal line 32 of the rod-like coupler are arranged in the uniform medium, which results in natural high directivity. As illustrated in FIG. 3, one of two metal rods which is a straight rod acts as the primary signal line while the other U-shaped rod welled on a PCB 33 acts as the coupled signal line, and 301, 302, 304 and 304 denote a first port, a second port, a third port and a fourth port, respectively.
For the metal rod coupler with the medium of air, relative positions of the two metal rods subject to an assembling precision thereof may further influence a coupling degree and a directivity index of the coupler, so that the assembled coupler may suffer from poor consistency of the directivity index and thus has to be connected to an external adjusting element.
A second prior art relates to a hanging wire leap-line coupler shown in FIG. 4a. As illustrated in FIG. 4a, its primary signal line 41 and coupled signal line 42 are composed of (rod-like) strip lines which are also substantially in the uniform medium of air, thereby resulting in natural high directivity; 401, 402, 404 and 404 denote a first port, a second port, a third port and a fourth port, respectively; and 43 denotes a PCB board.
There is another improved hanging wire leap-line coupler as illustrated in FIG. 4b which is different from that in FIG. 4a in that the hanging wire is replaced with a metal film resistor 44 inserted through a via hole and the body of the resistor function as a match load of the coupler.
Regardless of the hanging wire leap-line coupler in FIG. 4a or the improved hanging wire leap-line coupler in FIG. 4, they enjoy a slightly superior directivity index to that in the first prior art but may still suffer from poor consistency of the directivity index due to an accumulative error of the assembling precision. The coupler with good consistency of the directivity index has to be obtained at a relatively high cost.
A third prior art relates to a microstrip line directional coupler. A primary signal line and a coupled signal line of a conventional strip directional coupler are composed of strips, and the coupler is arranged in a non-uniform medium and thus has a poor directivity index. As illustrated in FIG. 5, the coupled signal line is arranged in a zigzag or wall buttress form to improve the directivity index by making the phase velocities of odd and even modes equal. A power capacity of the microstrip line directional coupler is far below those of the directional couplers in the first and second prior art. Further, the primary signal line of the coupler may suffer from a poor index of Passive Intermodulation (PIM) due to a large number of welding points.
A fourth prior art relates to an existing directional coupler illustrated in FIG. 7 in which a primary signal line is composed of a metal rod and a coupled signal line is composed of a microstrip line on a PCB, which are arranged, typically rectilinearly, in a non-uniform medium. The directional coupler structure in FIG. 7 has advantages of easy assembling and good consistency but may suffer from a poor directivity index of the coupler, approximately 15 dB or worse. Consequently, this coupler may be used only in a power detection circuit but not in a standing wave detection circuit.
Odd and even mode electromagnetic waves (simply referred to as odd and even modes hereinafter) are explained briefly here to facilitate better understanding of the disclosure later.
FIG. 6 illustrates a sectional view of a typical metal rod coupler structure in the prior art in which a backflow ground plane, a primary signal line, a coupled signal line and a backflow ground plane are shown from the top down with the medium of air arranged between the two ground planes. Generally, an odd mode is present between the primary signal line and the coupled signal line and an even mode is present in the entire cavity between the two backflow ground planes.
A phase velocity of either of the odd and even modes is dependent upon the nature of a medium in which the mode propagates. As well known, an electromagnetic wave propagates in the air at the velocity of light, and therefore both the phase velocity of the odd mode and that of the even mode are equal to the velocity of light.
Differently in the scenario of FIG. 7 where the coupled signal line in FIG. 7 is composed of the microstrip line on the PCB, the odd mode is still present in the medium of air and therefore the phase velocity of the odd mode is still approximate to the velocity of light, while a part of the even mode is present in the medium of PCB in which this part of electromagnetic wave propagates slowly and therefore the phase velocity of the odd mode is reduced throughout the system to the extent determined by the value of an equivalent dielectric constant of all the mediums between the two ground planes.
The fast odd mode and the slow even mode may result in the poor directivity index of the coupler, and an increase in the phase velocity of the even mode may be impossible due to the structure of the coupler.
Summarily for above, the directional couplers in the prior art may not be satisfactory in terms of all the parameter indexes such as the directivity index, consistency of the directivity index, the PIM index, the power capacity index, etc., and one or more of the parameter indexes of the existing directional couplers have to be improved in the prior art with a demanding precision and a high cost. Consequently, the parameter indexes of the existing directional couplers may not be improved effectively at a low cost.